Adds working version of bib entry mapping

o Uses setAs now as it is a coercion method
o See working example below.

```coffee
b <- new("article")
c <- new("ListOfcreator", firstName = "test")
j <- new("journal", journal = "Testjournal")
v <- new("volume", volume = "1")

b@creator = c
b@title = "the title"  # So that the entry isn't empty
b@journal = j
b@pubDate = "Test"

b@volume = v

as(b, "XMLInternalElementNode")
theentry = as(b, "bibentry")

print(theentry)

print(theentry, style = "citation")
print(theentry, style = "html")
print(theentry, style = "bibtex")
print(theentry, style = "R")
```

Belongs to issue #27
Merge pull request #74 from cpfaff/litrature

Adds a mostly working version of bib entry mapping for article